Windows 7 Pro
Intel Core i7-4900MQ
Worked: afaik never
Short description of error
Using the mirror modifier on a mesh with custom normals, the normals are not mirrored correctly, but are rotated all over the place, giving ugly render artifacts.
I work on heavy triangle meshes imported from other programs, and I often use the YAVNE addon to correct the normals (usually using "face area" for weighing). Other times I import FBX files including their custom normals. I do not want to double the geometry, so I use (or would like to use) the mirror modifier on all symmetrical parts. But this throws off their normals. And I also cannot just mirror them in edit mode (CTRL-M).
I have read somewhere that this behaviour if known. It does however look like a bug, and can destroy models if users are not aware (since custom normals data, once lost, cannot be reaquired).
In fact, I do not know of a way around the problem. How do I mirror custom normals? Maybe there is a way to fix this?
Exact steps for others to reproduce the error
I have attached a small file. The original object looks OK, the mirrored side looks distorted.